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The site is pretty fast. It states it's hosted by Clojure. I wonder what's the architecture layout is.

Whilst it doesn't tell you about the stack it's running on, the site code is open source: https://github.com/4clojure/4clojure

Looks like mongodb + compojure + some other interesting libraries (clojail for protected eval, for example), running on a Linode.

This is correct. It is worth noting that our setup is hardly complex or clever.